Low tension piston rings and method for manufacturing the same

ABSTRACT

The invention provides a low tension piston ring having a finished outer diameter and negligible tangential tension. The method includes the steps of machining a stock bar to an initial outer diameter slightly greater than the finished outer diameter of the piston ring, finishing the initial outer diameter of the stock bar to a rounded profile having a nominal diameter equal to the finished outer diameter, and machining the stock bar to the preferred cross-section. Dykes-type piston rings can have keystone or semi-keystone shaped cross-sections. The method continues by detaching the piston ring from the stock bar using a parting tool in response to completing all tension inducing operations including the steps recited above. The method continues with the steps of lapping the piston ring to a final longitudinal thickness and cutting the piston ring longitudinally to form a final gap.

This Divisional patent application claims priority to U.S. Utilityapplication Ser. No. 13/827,255, filed Mar. 14, 2013, and isincorporated herein by reference.

TECHNICAL FIELD

The subject invention provides a method for manufacturing a low tensionpiston ring, specifically a dykes-type piston ring having a finishedouter diameter and negligible tangential tension.

BACKGROUND

Piston rings are a critical component of an internal combustion engine.The engine includes at least one cylinder and piston. Piston rings aremetallic seals disposed between the cylinder wall and the piston to sealthe combustion chamber from the crankcase and facilitate heat transferfrom the piston to the cylinder. Other functions of piston rings are toprevent the oil not needed for lubrication from passing from thecrankcase to the combustion chamber and to provide a uniform oil film onthe cylinder bore surface. To achieve this, the piston rings must remainin contact with the cylinder and the piston. Radial contact is generallyachieved by means of the inherent spring force of the piston ring.Piston rings are also employed as metallic seals for rotating shafts andare used both as contracting and expanding seals.

Today, piston rings are typically manufactured in one of two ways. Inone method, the piston rings are cast as individual rings in anoncircular shape. Such rings are then typically machined to therequired shape by means of double cam turning, a process in which thering blank, already axially ground, is cam turned simultaneously on theinside and outside diameters. After a segment equivalent to the free gapis cut from the piston ring, it assumes the free shape that will give itthe required radial pressure distribution when fitted into the cylinder.Once inside the cylinder, the piston ring exerts the predefined radialpressure against the cylinder wall. Besides using double cam turning,ring blanks may also be shaped by machining the inside and outsidediameters separately. This involves cam turning the outside diameter ofthe noncircular blank and machining the inside diameter with the pistonring in the compressed state. The free gap is cut out in a step betweenthe outer diameter and inner diameter machining.

According to a different method, steel piston rings are made from aprofiled wire. The rings are first coiled into a circular shape and thenthe gap is cut out. The necessary shape is obtained using a heattreatment process in which the rings are mounted onto an arborappropriately designed to impart the required radial pressuredistribution. Profiling of the outer diameter is carried out, dependingon the piston ring design, on automatic outer diameter lathes or profilegrinding machines using profile cutting tools.

The problem with manufacturing piston rings according to the methodsdescribed above is that the piston rings are produced with residualtangential tension. Production of piston rings with residual tangentialtension is problematic because such piston rings have a tendency totwist or warp. Such ring twist or warp may lead to excessive oilconsumption and blow-by, a condition where combustion gasses escape fromthe combustion chamber by passing along the piston between the pistonrings and the cylinder wall. Accordingly, piston rings with residualtangential tension may adversely affect the efficiency, the performance,the emissions, and/or the reliability of the engine.

Methods aimed at producing piston rings with reduced tendency to twistor warp have been developed. One such method includes the step of heattreating a stock bar made of cast iron at a high temperature, forexample 1100° Fahrenheit (593.33° Celsius), to remove foundry strainsand hard spots. After the stock bar is heat treated, piston ring blanksare cut from the stock bar. Once the piston ring blanks are detachedfrom the stock bar, the method continues with the steps of conventionalmachining and finishing the piston ring blank to final outer and innerdiameters. The last step is cutting a final free gap into the pistonring. Although this method produces rings having less of a tendency totwist or warp, significant tangential tension still remains in thepiston rings.

What is needed is a method of manufacturing piston rings wherein only anegligible amount of residual tangential tension remains in the finishedproducts. Without residual tension, the rings will not have a tendencyto twist or warp. Residual tension is especially a concern with pistonrings with unconventional cross-sections, such as dyke-type piston ringswhich have an L-shaped cross-section.

Dykes type piston rings allow for better sealing at higher engine speedsand combustion pressures. However, the asymmetrical shape of the dykestype piston ring results in a piston ring that is more prone to twist orwarp than conventional piston rings. Accordingly, there is a need forpiston rings, particularly dykes-type piston rings with negligibletangential tension.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the invention, a pistonring is detached from a stock bar using a parting tool after completingall tension inducing operations including the steps of machining thestock bar to the initial outer diameter, finishing the stock bar to apredetermined profile, and machining the stock bar to a predeterminedcross-section. The piston ring is not separated from the stock bar untilafter all of the machining and finishing steps have been completed. Inthis manner, the stresses associated with the machining and finishingoperations are borne by the thicker stock bar before the piston ring isseparated.

As to dykes-type piston rings in particular, the subject invention canprovide a dykes-type piston ring including an annular ring body having ameasurable tangential tension ranging from zero to 25 Newtons in a freeand uncompressed state. In accordance with this design, the dykes typepiston ring defines a final gap having a gap width measurable between apair of lateral faces ranging from zero to 0.4 millimeters when theannular ring body is in its free and uncompressed state.

These and other features and advantages of the present invention will bereadily appreciated, as the same becomes better understood by referenceto the following detailed description when considered in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary dykes-type piston ring.

FIG. 2 is a cross-sectional view of the exemplary dykes-type pistonring.

FIG. 3 is a perspective view of an exemplary dykes-type piston ringexperiencing a condition known as piston ring warp or piston ring twist.

FIGS. 4-6 are perspective views illustrating steps of machining a stockbar to form a piston ring.

FIG. 7 is a perspective and elevation view illustrating an exemplarystep of machining the stock bar to an L-shaped cross-section;

FIG. 8 is a perspective and elevation view illustrating an exemplarystep of detaching the piston ring from the stock bar;

FIG. 9 is a perspective view illustrating an exemplary step of lappingthe piston ring to a final longitudinal thickness;

FIG. 10 is a perspective and elevation view illustrating an exemplarystep of cutting the piston ring longitudinally to form a final gap; and

FIG. 11 is a perspective view illustrating an exemplary step ofinstalling the piston ring on a mandrel to size the final gap to apredetermined dimension during heat treating.

FIGS. 12 and 13 illustrate exemplary pistons with traditional pistonrings and with dykes-type piston rings, respectively.

DETAILED DESCRIPTION OF THE EMBODIMENT

FIG. 12 illustrates a conventional piston 100 with traditional pistonrings 18 mounted on it. FIG. 13 illustrates a piston 200 with dykes-typepiston rings 20 positioned on it.

In the Figures, like numerals indicate corresponding parts throughoutthe several views. In FIGS. 1 and 2, a dykes-type piston ring 20 havinga finished outer diameter is presented. It should be appreciated thatthe piston ring 20 is circular in shape and has a circumference 24. Thefinished outer diameter extends along the circumference 24.

A preferred method embodiment of the subject invention is shown in FIGS.4-6. The method includes the steps of machining a stock bar 26 to aninitial outer diameter 28 slightly greater than the finished outerdiameter of the piston ring 20, finishing the initial outer diameter ofthe stock bar 26 to a predetermined profile 30 having a nominal diameter32. The diameter 32 is equal to the finished outer diameter 22. Thestock bar 26 is also machined to form a structure with an L-shapedcross-section. The method continues with the step of detaching thepiston ring 20 from the stock bar 26 in response to completing alltension inducing operations including the steps of machining the stockbar 26 to the initial outer diameter 28, finishing the stock bar 26 to apredetermined profile 30, and machining the stock bar 26 to an L-shapedcross-section 34.

It has been found that machining and finishing operations typicallyleave residual tensile forces in thin work pieces such as piston rings.This residual tension arises in a tangential direction with respect tothe circumference of the piston rings. As a result, the piston rings areprone to twist or warp around the circumference when constrained in thecylinder of an internal combustion engine. This is shown in exaggeratedform in FIG. 3.

With the present invention where the piston ring is not separated fromthe stock bar 26 until after all of the machining and finishing stepshave been completed, the stresses associated with the machining andfinishing operations are borne by the thicker and more rigid stock bar26. It has been found that the tangential tension induced by themachining and finishing operations can be better controlled andminimized in this manner. As a result, the piston ring 20 has only anegligible amount of tangential tension when it is detached from thestock bar 26 in comparison to the prior art.

Piston ring twist or warp is particularly harmful in applications thatutilize dykes-type piston rings due to the asymmetrical (“L-shaped”)cross-sectional shapes of the rings.

According to one aspect of the subject invention for manufacturingdykes-type piston rings, the method step of machining the stock bar toan L-shaped cross-section may include machining a lip section extendinglongitudinally and a tail section extending radially inwardly from thelip section. The tail section may be machined to present a keystoneshaped cross section defined by one or two inwardly tapering sides.

The method of finishing the initial outer diameter of a stock bar to apredetermined profile having a nominal diameter equal to the finishedouter diameter may include finishing the initial outer diameter of thestock bar to a predetermined profile having a variety of shapes. Thepredetermined profile may have a rounded shape. With a symmetrical orasymmetrical curve profile. For example, as shown in FIG. 6, theexemplary rounded profile asymmetrical curve profile can be defined by apair of arcs 42 converging at a tangent point 44. The arcs 42 may havedifferent radii. Alternatively, the predetermined profile may have aflat linear shape arranged along the longitudinal direction or sloped atan angle relative to the longitudinal direction. Regardless of the shapeof the predetermined profile, the term “nominal diameter”, as it is usedherein, designates the maximum outer diameter of the predeterminedprofile.

In one preferred embodiment, the step of machining a stock bar 26 to aninitial outer diameter 28 slightly greater than the finished outerdiameter 22 of the piston ring 20 may include machining the stock bar 26such that the initial outer diameter 28 is between 0.4 mm and 3.0 mmlarger than the finished outer diameter 22 of the piston ring 20. Inthis way, the profiling step may be completed more quickly as lessmaterial is removed by profiling when the initial outer diameter 28 ofthe stock bar 26 is close to the finished outer diameter 22 of thepiston ring 20. Additionally, the profiling step induces less tangentialtension on the stock bar 26 when only a small thickness of material hasto be removed.

The method may further include the steps of lapping the piston ring 20to a final longitudinal thickness and cutting the piston ring 20longitudinally to form a final gap 50. It should be appreciated that thelapping step may include rubbing the piston ring 20 with an abrasive.The lapping step may include using a dual action lapping machine (notshown) having a rotating lapping pad presenting an abrasive surface. Thepiston ring and the lapping pad may be rotated in opposite directionsand the piston ring 20 may be reciprocated back and forth in atransverse direction across the rotating lapping pad.

As shown in FIGS. 9 and 10, the cutting step may include using acircular saw blade 56 to make a longitudinal cut through the entireL-shaped cross-section of the piston ring 20 to form the final gap 50.The final gap 50 need only be sufficient in size so as to prevent endbutting of the piston ring 20 at the operational temperatures of theengine. Accordingly, the final gap 50 of the dykes-type piston ring 20produced by this method may be many times smaller than the final gap ofconventional piston rings.

In accordance with the subject invention, the method may further includethe step of heat treating the piston ring 20 by placing the piston ring20 in a non-oxidizing atmosphere between 400-450° Celsius to relieve anyresidual tension left in the piston ring 20. The step of heat treatingthe piston ring 20 in a non-oxidizing atmosphere may be achieved bybaking the piston ring 20 in an oven filled with a non-oxidizing gassuch as nitrogen. The method may further include the step of installingthe rings on a mandrel 58 to size the final gap 50 to a predetermineddimension during the heat treating step. This is shown in FIG. 11.

The method step of machining the stock bar to the initial outer diametermay include turning the stock bar relative to a machining tool.Similarly, the step of finishing the initial outer diameter to apredetermined profile may include turning the stock bar 26 relative to aprofiling tool 62 (FIG. 5). The step of machining the stock bar 26 tothe L-shaped cross section may include turning the stock bar 26 relativeto a machining tool 60, and the step of detaching the piston ring 20from the stock bar 26 may include turning the stock bar 26 relative to aparting tool 64 (FIG. 8). As the term is used herein, turning is aspecific machining process wherein the piston ring is rotated relativeto the tool using a lathe. It should be appreciated that the machining,finishing, and parting steps provided are not limited to turning and canbe accomplished using a variety of other tools and operations.

A variety of materials have been found to be suitable for themanufacture of piston rings 20. According to an aspect of the subjectinvention, the stock bar 26 can be made of a fine grained spheroidalcast iron. However, it is envisioned that other materials may be used.

It should be appreciated that the method of the subject inventionapplies not only to the manufacture of low tension dykes-type pistonrings, but to the manufacture of other types of low tension piston ringsas well. Accordingly, a method for manufacturing a low tension pistonring having a finished outer diameter is also provided. The methodincludes the steps of machining a stock bar to an initial outer diameterslightly greater than the finished outer diameter of the piston ring,finishing the initial outer diameter of the stock bar to a predeterminedprofile having a nominal diameter equal to the finished outer diameter,and machining the stock bar to a predetermined cross-section. The methodcontinues with the step of detaching the piston ring from the stock barin response to completing all tension-inducing operations. Theseoperations include machining the stock bar to an initial outer diameter,finishing the stock bar to a predetermined profile, and machining thestock bar to a predetermined cross-section. According to one aspect ofthis method, the step of machining a stock bar to an initial outerdiameter slightly greater than the finished outer diameter of the pistonring may include finishing the initial outer diameter of the stock barto a predetermined profile having a rounded shape. According to anotheraspect of this method, the step of machining the stock bar to apredetermined cross-section may include machining the stock bar to akeystone shaped cross-section or a semi-keystone shaped cross-section.

The method described above produces a novel dykes-type piston ring 20having unique properties. As shown in FIGS. 1 and 2, the dykes-typepiston ring 20 that is produced includes an annular ring body 68presenting a generally L-shaped cross-section 34. The L-shapedcross-section 34 is comprised of and defined by a lip section 36extending longitudinally relative to the ring body 68 and a tail section38 having a pair of sides 40 extending radially inwardly from the lipsection 36. The annular ring body 68 also presents a pair of lateralfaces 70 defining a final gap 50 therebetween. It should be appreciatedthat the final gap 50 is created by the method step of cutting thedetached piston ring 20 longitudinally or discussed above. The pair oflateral faces 70 is arranged such that they face one another in anopposing spaced relationship. The annular ring body 68 has little or notangential tension, ranging from zero to 25 Newtons (N), in a free anduncompressed state. This range is considerably less than the tangentialtension residing in piston rings 20 produced by prior art methods. Thislow tangential tension also allows the piston ring 20 to have aminiscule final gap 50 in comparison to conventional piston ringsincluding prior art dykes-type piston rings. In accordance with thesubject invention, the final gap 50 has a gap width 72 measurablebetween the lateral faces 70 ranging from zero to 0.4 millimeters whenthe annular ring body 68 is in its free and uncompressed state. Inaccordance with another aspect of the subject invention, the gap width72 may range from zero to 0.1 millimeters when the annular ring body 68is in its free and uncompressed state. These ranges are approximatelythirty times smaller than the final gap of conventional piston rings.

The dykes-type piston ring 20 of the subject invention may furtherinclude a tail section 38 having various shapes. The side of the tailsection 38 may each tapper inwardly to define a keystone shapedcross-section. Alternatively, one of the sides of the tail section maytapper inwardly to define a semi-keystone shaped cross-section. Thedykes-type piston ring 20 of the subject invention may further includean outer circumferential surface having a predetermined profile ofarcuate shape. The arcuate, or rounded, shape of the predeterminedprofile may follow a symmetrical or asymmetrical curve. For example, thepredetermined profile may follow an asymmetrical curve as shown in FIGS.5 and 6 where the curve is defined by a pair of arcs 42 converging at atangent point 44. The arcs 42 can have different radii 46.Alternatively, the predetermined profile 30 may have a flat linear shapearranged along the longitudinal direction or sloped at an angle relativeto the longitudinal direction.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings and may be practicedotherwise than as specifically described while within the scope of theappended claims.

What is claimed is:
 1. A dykes-type piston ring comprising; an annularring body presenting a generally L-shaped cross-section with a lipsection that extends longitudinally relative to said ring body and atail section with a pair of sides that extends radially inwardly fromsaid lip section, said annular ring body having a pair of lateral facesdefining a final gap therebetween, and said annular ring body having ameasurable tangential tension ranging from zero to 25 Newtons in a freeand uncompressed state.
 2. A dykes type piston ring as set forth inclaim 1 wherein said final gap has a gap width measurable between saidlateral faces ranging from zero to 0.4 millimeters when said annularring body is in said free and uncompressed state.
 3. A dykes type pistonring as set forth in claim 2 wherein said gap width ranges from zero to0.1 millimeters when said annular ring body is in said free anduncompressed state.
 4. A dykes type piston ring as set forth in claim 1wherein said pair of sides of said tail section each tapper inwardly todefine a keystone shaped cross-section.
 5. A dykes type piston ring asset forth in claim 1 wherein one of said pair of sides of said tailsection tappers inwardly to define a semi-keystone shaped cross-section.6. A dykes type piston ring as set forth in claim 1 wherein said annularring body includes an outer circumferential surface having apredetermined profile of arcuate shape.